共查询到20条相似文献,搜索用时 31 毫秒
1.
Buoyant flows often contain regions with unstable and stable thermal stratification from which counter gradient turbulent
fluxes are resulting, e.g. fluxes of heat or of any turbulence quantity. Basing on investigations in meteorology an improvement
in the standard gradient-diffusion model for turbulent diffusion of turbulent kinetic energy is discussed. The two closure
terms of the turbulent diffusion, the velocity-fluctuation triple correlation and the velocity-pressure fluctuation correlation,
are investigated based on Direct Numerical Simulation (DNS) data for an internally heated fluid layer and for Rayleigh–Bénard
convection. As a result it is decided to extend the standard gradient-diffusion model for the turbulent energy diffusion by
modeling its closure terms separately. Coupling of two models leads to an extended RANS model for the turbulent energy diffusion.
The involved closure term, the turbulent diffusion of heat flux, is studied based on its transport equation. This results
in a buoyancy-extended version of the Daly and Harlow model. The models for all closure terms and for the turbulent energy
diffusion are validated with the help of DNS data for internally heated fluid layers with Prandtl number Pr = 7 and for Rayleigh–Bénard convection with Pr = 0.71. It is found that the buoyancy-extended diffusion model which involves also a transport equation for the variance
of the vertical velocity fluctuation gives improved turbulent energy diffusion data for the combined case with local stable
and unstable stratification and that it allows for the required counter gradient energy flux. 相似文献
2.
M. Klein 《Flow, Turbulence and Combustion》2005,75(1-4):131-147
While methods for assessing the uncertainty of Reynolds–Averaged–Navier–Stokes (RANS) simulations have been well established
in the past, the verification of Large Eddy Simulations (LES) is more difficult. One reason is that the numerical discretization
error as well as the subgrid scale model contribution depend on the grid resolution and that both terms interact. In the present
paper the accuracy of single-grid estimators to assess the amount of the unresolved turbulent kinetic energy is studied first.
In the second part of the paper the sensitivity of the simulation results on the modeling error as well as the numerical error
will be investigated in the context of LES with implicit filtering. This will be achieved by performing a systematic grid
and model variation. The analysis is applied to an isothermal, turbulent, plane jet and a turbulent channel flow. 相似文献
3.
Fujihiro Hamba 《Theoretical and Computational Fluid Dynamics》2001,14(5):323-336
Large eddy simulation (LES) is combined with the Reynolds-averaged Navier–Stokes (RANS) equation in a turbulent channel-flow
calculation. A one-equation subgrid-scale model is solved in a three-dimensional grid in the near-wall region whereas the
standard k–ε model is solved in a one-dimensional grid in the outer region away from the wall. The two grid systems are overlapped to
connect the two models smoothly. A turbulent channel flow is calculated at Reynolds numbers higher than typical LES and several
statistical quantities are examined. The mean velocity profile is in good agreement with the logarithmic law. The profile
of the turbulent kinetic energy in the near-wall region is smoothly connected with that of the turbulent energy for the k–ε model in the outer region. Turbulence statistics show that the solution in the near-wall region is as accurate as a usual
LES. The present approach is different from wall modeling in LES that uses a RANS model near the wall. The former is not as
efficient as the latter for calculating high-Reynolds-number flows. Nevertheless, the present method of combining the two
models is expected to pave the way for constructing a unified turbulence model that is useful for many purposes including
wall modeling.
Received 11 June 1999 and accepted 15 December 2000 相似文献
4.
P. G. Zaets A. F. Kurbatskii A. T. Onufriev S. V. Poroseva N. A. Safarov R. A. Safarov S. N. Yakovenko 《Journal of Applied Mechanics and Technical Physics》1998,39(2):249-260
The vorticity formed in the cross section of a turbulent flow in a straight circular pipe rotating about its longitudinal
axis decreases the values of the turbulent stresses, turbulence energy, and dissipation rate along the pipe. The results of
laboratory experiments and calculations by the second-order closure model of turbulent transfer are presented. On the whole,
the model using a system of transport equations yields better agreement with experimental data than the models with algebraic
relations for second-order moments.
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 39, No. 2, pp. 103–116, March–April, 1998. 相似文献
5.
A turbulent wake model, based on the Reynolds, energy and turbulence dissipation equations together with the closing relations
for the turbulent transport coefficients, is proposed. A comparative investigation of swirled momentumless wakes with zero
and nonzero angular momentum is carried out.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 13–22, January–February,
1994. 相似文献
6.
I. V. Derevich 《Fluid Dynamics》1998,33(4):497-511
Using a two-point probability density function for the particle distribution over velocities and coordinates, a closed model
of the particle effect on the turbulent flow characteristics is formulated. The processes of turbulent dissipation and turbulent
energy transfer across the spectrum are studied. Different models of two-phase turbulence are compared.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 40–56, July–August, 1998.
The work received financial support from INTAS (grant No. 94-4348) and the Russian Foundation for Basic Research (project
No. 98-01-00-353). 相似文献
7.
The results of the numerical simulation of supersonic three-dimensional flow past sharp-nosed cones with circular and elliptic
cross-sections in the turbulent shock-layer flow regime are presented. The calculations are performed in the local conical
approximation using the system of Reynolds equations and the differential one-equation turbulence model. The numerical solutions
are obtained by means of an implicit constant-direction finite-difference scheme. The emphasis is placed on the investigation
of the transverse flow separation and the flow features associated with the turbulent flow regime.
St.Petersburg. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 99–105, January–February,
2000.
The study was carried out with the support of the Russian Foundation for Basic Research (project No. 99-01-00735). 相似文献
8.
C.G. Speziale 《Theoretical and Computational Fluid Dynamics》1999,13(3):161-166
A resent extension of the nonlinear K–ε model is critically discussed from a basic theoretical standpoint. While it was said in the paper that this model was formulated
to incorporate relaxation effects, it will be shown that the model is incapable of describing one of the most basic such turbulent
flows as is obvious but is described for clarity. It will be shown in detail that this generalized nonlinear K–ε model yields erroneous results for the Reynolds stress tensor when the mean strains are set to zero in a turbulent flow
– the return-to-isotropy problem which is one of the most elementary relaxational turbulent flows. It is clear that K–ε type models cannot describe relaxation effects. While their general formalism can describe relaxation effects, the nonlinear
K–ε model – which the paper is centered on – cannot. The deviatoric part of the Reynolds stress tensor is predicted to be zero
when it actually only gradually relaxes to zero. Since this model was formulated by using the extended thermodynamics, it
too will be critically assessed. It will be argued that there is an unsubstantial physical basis for the use of extended thermodynamics
in turbulence. The role of Material Frame-Indifference and the implications for future research in turbulence modeling are
also discussed.
Received 19 February 1998 and accepted 23 October 1998 相似文献
9.
Yu. A. Gostintsev V. V. Lazarev A. F. Solodovnik Yu. V. Shatskikh 《Fluid Dynamics》1986,21(6):965-976
An approximate analytical model of a turbulent thermal in a stratified atmosphere is proposed. This model makes it possible
to predict the dynamics of the ascent, suspension and oscillation processes of a buoyant cloud both within the troposphere
and on entering the stratossphere. The values of the heat energy needed for the thermal to penetrate the tropopause in northern
and southern latitudes are estimated. Estimates are obtained for the amount of material dumped into the stratosphere. A method
of determining the thermal energy of volcanic eruptions of the explosive type is proposed.
Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 141–153, November–December, 1986. 相似文献
10.
W. D. Thacker S. Sarkar T. B. Gatski 《Theoretical and Computational Fluid Dynamics》2007,21(3):171-199
The influence of compressibility on the rapid pressure–strain rate tensor is investigated using the Green’s function for the
wave equation governing pressure fluctuations in compressible homogeneous shear flow. The solution for the Green’s function
is obtained as a combination of parabolic cylinder functions; it is oscillatory with monotonically increasing frequency and
decreasing amplitude at large times, and anisotropic in wave-vector space. The Green’s function depends explicitly on the
turbulent Mach number M
t
, given by the root mean square turbulent velocity fluctuations divided by the speed of sound, and the gradient Mach number
M
g
, which is the mean shear rate times the transverse integral scale of the turbulence divided by the speed of sound. Assuming
a form for the temporal decorrelation of velocity fluctuations brought about by the turbulence, the rapid pressure–strain
rate tensor is expressed exactly in terms of the energy (or Reynolds stress) spectrum tensor and the time integral of the
Green’s function times a decaying exponential. A model for the energy spectrum tensor linear in Reynolds stress anisotropies
and in mean shear is assumed for closure. The expression for the rapid pressure–strain correlation is evaluated using parameters
applicable to a mixing layer and a boundary layer. It is found that for the same range of M
t
there is a large reduction of the pressure–strain correlation in the mixing layer but not in the boundary layer. Implications
for compressible turbulence modeling are also explored.
相似文献
11.
The results of calculating the diffusion of a dispersed admixture in turbulent swirling jet flows using the model of momentum
transfer in a turbulent gas—dispersion flow proposed by the authors are presented. These results are compared with experimental
data and with calculations based on various mathematical models.
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No.1, pp. 71–78, January–February,
1994. 相似文献
12.
The propagation mechanism of high speed turbulent deflagrations 总被引:2,自引:0,他引:2
The propagation regimes of combustion waves in a 30 cm by 30 cm square cross–sectioned tube with an obstacle array of staggered
vertical cylindrical rods (with BR=0.41 and BR=0.19) are investigated. Mixtures of hydrogen, ethylene, propane, and methane with air at ambient conditions over a range
of equivalence ratios are used. In contrast to the previous results obtained in circular cross–sectioned tubes, it is found
that only the quasi–detonation regime and the slow turbulent deflagration regimes are observed for ethylene–air and for propane–air.
The transition from the quasi–detonation regime to the slow turbulent deflagration regime occurs at (where D is the tube “diameter” and is the detonation cell size). When , the quasi–detonation velocities that are observed are similar to those in unobstructed smooth tubes. For hydrogen–air mixtures,
it is found that there is a gradual transition from the quasi–detonation regime to the high speed turbulent deflagration regime.
The high speed turbulent deflagration regime is also observed for methane–air mixtures near stoichiometric composition. This
regime was previously interpreted as the “choking” regime in circular tubes with orifice plate obstacles. Presently, it is
proposed that the propagation mechanism of these high speed turbulent deflagrations is similar to that of Chapman–Jouguet
detonations and quasi-detonations. As well, it is observed that there exists unstable flame propagation at the lean limit
where . The local velocity fluctuates significantly about an averaged velocity for hydrogen–air, ethylene–air, and propane–air mixtures.
Unstable flame propagation is also observed for the entire range of high speed turbulent deflagrations in methane–air mixtures.
It is proposed that these fluctuations are due to quenching of the combustion front due to turbulent mixing. Quenched pockets
of unburned reactants are swept downstream, and the subsequent explosion serves to overdrive the combustion front. The present
study indicates that the dependence on the propagation mechanisms on obstacle geometry can be exploited to elucidate the different
complex mechanisms of supersonic combustion waves.
Received 5 November 2001 / Accepted 12 June 2002 / Published online 4 November 2002
Correspondence to: J. Chao (e-mail: jenny.chao@mail.mcgill.ca)
An abridged version of this paper was presented at the 18th Int. Colloquium on the Dynamics of Explosions and Reactive Systems
at Seattle, USA, from July 29 to August 3, 2001. 相似文献
13.
We consider non-linear viscous shallow water models with varying topography, extra friction terms and capillary effects, in
a two-dimensional framework. Water-depth dependent laminar and turbulent friction coefficients issued from an asymptotic analysis
of the three-dimensional free-surface Navier–Stokes equations are considered here. A new proof of stability for global weak
solutions is given in periodic domain Ω = T2, adapting the method introduced by J. Simon in [15] for the non-homogeneous Navier–Stokes equations. Existence results for
such solutions can be obtained from this stability analysis. 相似文献
14.
V. P. Reutov A. B. Ezersky G. V. Rybushkina V. V. Chernov 《Journal of Applied Mechanics and Technical Physics》2007,48(4):469-478
Evolution of convective structures in a thin layer of an evaporating liquid (ethanol) located under a turbulent boundary layer
of an airflow is studied experimentally and theoretically. Evolution of the structures is examined under conditions of an
increased flow velocity. A transition is found from convective cells formed in the absence of the flow to convective rolls
elongated in the streamwise direction. The theoretical analysis is performed within a two-dimensional model of the flow in
the liquid layer. The boundary conditions on the liquid surface are obtained with the use of self-similar solutions for mean
fields in the airflow. The onset and evolution of a periodic system of rolls are simulated numerically. Theoretical conclusions
are compared with experimental data.
__________
Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 4, pp. 3–14, July–August, 2007. 相似文献
15.
Most gas dynamic computations in industrial ducts are done in one dimension with cross-section-averaged Euler equations. This
poses a fundamental difficulty as soon as geometrical discontinuities are present. The momentum equation contains a non-conservative
term involving a surface pressure integral, responsible for momentum loss. Definition of this integral is very difficult from
a mathematical standpoint as the flow may contain other discontinuities (shocks, contact discontinuities). From a physical
standpoint, geometrical discontinuities induce multidimensional vortices that modify the surface pressure integral. In the
present paper, an improved 1D flow model is proposed. An extra energy (or entropy) equation is added to the Euler equations
expressing the energy and turbulent pressure stored in the vortices generated by the abrupt area variation. The turbulent
energy created by the flow–area change interaction is determined by a specific estimate of the surface pressure integral.
Model’s predictions are compared with 2D-averaged results from numerical solution of the Euler equations. Comparison with
shock tube experiments is also presented. The new 1D-averaged model improves the conventional cross-section-averaged Euler
equations and is able to reproduce the main flow features. 相似文献
16.
The motivation of this study is to investigate the turbulence–chemistry interactions by using probability density function
(PDF) method. A consistent hybrid Reynolds Averaged Navier–Stokes (RANS)/PDF method is used to simulate the turbulent non-reacting
and reacting flows. The joint fluctuating velocity–frequency–composition PDF equation coupled with the Reynolds averaged density,
momentum and energy equations are solved on unstructured meshes by the Lagrangian Monte Carlo (MC) method combined with the
finite volume (FV) method. The simulation of the axisymmetric bluff body stabilized non-reacting flow fields is presented
in this paper. The calculated length of the recirculation zone is in good agreement with the experimental data. Moreover,
the significant change of the flow pattern with the increase of the jet-to-coflow momentum flux ratio is well predicted. In
addition, comparisons are made between the joint PDF model and two different Reynolds stress models.
The project supported by the National Natural Science Foundation of China (50506028), and Action Scheme for Invigorating Education
Towards the twenty-first century. 相似文献
17.
We study the connection between atomistic and continuum models for the elastic deformation of crystalline solids at zero temperature.
We prove, under certain sharp stability conditions, that the correct nonlinear elasticity model is given by the classical
Cauchy–Born rule in the sense that elastically deformed states of the atomistic model are closely approximated by solutions
of the continuum model with stored energy functionals obtained from the Cauchy–Born rule. The analysis is carried out for
both simple and complex lattices, and for this purpose, we develop the necessary tools for performing asymptotic analysis
on such lattices. Our results are sharp and they also suggest criteria for the onset of instabilities of crystalline solids. 相似文献
18.
Dilute polymer solutions that reduce turbulent friction are treated as viscoelastic liquids for which, in addition to the
Reynolds number, the scale-up criteria include the Deborah or elasticity number. Introduction of a generalized (viscoelastic)
Reynolds number makes it possible to reduce the experimental curves for the drag coefficients obtained in pipes of various
diameters at different and concentrations to a unified dependence.
St. Petersburg. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 4, pp. 191–196, July–August,
1998. 相似文献
19.
The flow initiated by a hot gas cloud (thermal) in a stratified atmosphere is calculated on the basis of theκ-ε turbulence model and the transport model for the Reynolds stresses and turbulent fluxes and the results obtained are compared
The nonlocal nature of the turbulent transport in a vortex ring and its effect on certain flow characteristics are explained
In particular, the calculations carried out using the Reynolds stress model show much slower cooling of the temperature-vortex
torus than those based calculated on theκ-ε-model Modification of theκ-ε-model to take the effect of curvature of the streamlines approximately into account makes it only partially possible to reproduce
the results obtained on the basis of the Reynolds stress model
Moscow. Translated from Izvestiya Rossiiskoi Akademii Nauk, Mekhanika Zhidkosti i Gaza, No. 1, pp. 12–20, January–February,
1999.
The research was carried out with support from the Russian Foundation for Basic Research (project No. 95-01-00544a). 相似文献
20.
In the theory of nonlinear elasticity of rubber-like materials, if a homogeneous isotropic compressible material is described
by a strain–energy function that is a homogeneous function of the principal stretches, then the equations of equilibrium for axisymmetric deformations reduce to a separable
first-order ordinary differential equation. For a particular class of such strain–energy functions, this property is used
to obtain a general parametric solution to the equilibrium equation for plane strain bending of cylindrical sectors. Specification
of the arbitrary function that appears in such strain–energy functions yields some parametric solutions. In some cases, the
parameter can be eliminated to yield closed-form solutions in implicit or explicit form. Other possible forms for the arbitrary
constitutive function that are likely to yield such solutions are also indicated. 相似文献